How to change contact number

Hallo, ich hoffe, dass es OK ist, auf Englisch zu schreiben.
I have an appointment for DSL installation on 12.12, however the phone number in "Meine O2" is wrong, and I cannot change this. I have logged in to Meine O2 and selected "Login verwalten", but this does not allow me to change the phone number or add a new phone number. I need to change this number before 12.12 so the technician can contact me!
Bitte helfen!